A conventional object detecting apparatus mounted on a vehicle uses a laser light, for instance, to detect an object such as a preceding vehicle or a distance to such an object. This detecting apparatus intermittently drives a laser diode to radiate the laser light towards the forward area of the vehicle, and detects the light reflected from the forward obstacle by a photo sensor. The detecting apparatus measures the distance to the forward obstacle based on a time difference between a light radiation time and a light receiving time.
Specifically, as disclosed in JP 2002-031685A, the detecting apparatus comprises a light radiation unit for radiating a laser light, a polygon mirror and a light reception unit for receiving a reflected light. The polygon mirror is shaped in a frustum of a hexagonal pyramid and rotatable as a scanning mirror. According to this construction, the polygon mirror reflects the laser light radiated from the light radiation unit and directs it to the forward area of the vehicle.
As the polygon mirror is rotated and the laser light from the light radiation unit is directed to each side surface of the polygon mirror, so that the angle of reflection of the laser light at the polygon mirror is adjusted to scan a predetermined range of the forward area of the vehicle by the laser light. The reception unit includes a Fresnel lens and a light receiving element to receive the laser light reflected from the forward object and measure the distance to the object.
This detecting apparatus requires a light radiation and light reception monitoring function to check whether the laser light is radiated without fail and the laser light reflected by the forward object is received without fail.
For this function, the detecting apparatus is provided with a transparent panel inside a case to form a self-monitoring light path. Specifically, the laser radiation unit is driven to radiate the laser light when each side surface of the polygon mirror is directed to a specified direction different from the direction for scanning the predetermined range. This laser light is reflected by the transparent panel and the reflected light is received by the reception unit. Abnormality of the detecting apparatus is determined, if the reception unit does not produce an electric signal in spite of the laser light radiation by the radiation unit.
In the above arrangement, the reception unit is disposed side by side in the lateral or horizontal direction. Namely, the reception unit is disposed on a plane perpendicular to the axis of rotation of the polygon mirror to face the polygon mirror, that is, disposed in the direction to which the laser light reflected by the polygon mirror is angle-adjusted. Thus, the laser light reflected by the polygon mirror and led to the transparent panel is reflected by the transparent panel and led to the reception unit. However, this arrangement is not possible in the case that the reception unit is disposed side by side in the vertical direction and not in the horizontal direction.